Although Caesar portrayed this invasion as being a preemptive and defensive action, most historians agree that the wars were fought primarily to boost Caesar's political career and to pay off his massive debts. Still, Gaul was of significant military importance to the Romans, as they had been attacked several times by native tribes both indigenous to Gaul and farther to the north. Conquering Gaul allowed Rome to secure the natural border of the river Rhine. The Gallic Wars are described by Julius Caesar in his book Commentarii de Bello Gallico, which remains the most important historical source regarding the conflict.[1]

His ambition was to conquer and plunder some territories to get himself out of debt, and it is possible that Gaul was not his initial target. It is more likely that he was planning a campaign against the Kingdom of Dacia,[2] located in the Balkans.

The countries of Gaul were civilized and wealthy. Most had contact with Roman merchants and some, particularly those that were governed by republics such as the Aedui and Helvetii, had enjoyed stable political alliances with Rome in the past.

The Romans respected and feared the Gallic tribes. Only fifty years before, in 109 BC, Italy had been invaded from the north and saved only after several bloody and costly battles by Gaius Marius. Around 62 BC, when a Roman client state, the Arverni, conspired with the Sequani and the Suebi nations east of the Rhine, to attack the Aedui, a strong Roman ally, Rome turned a blind eye. The Sequani and Arverni sought Ariovistus’ aid and defeated the Aedui in 63 BC at the Battle of Magetobriga. The Sequani rewarded Ariovistus with land following his victory.[3][4][5] Ariovistus settled the land with 120,000 of his people. When 24,000 Harudes joined his cause, Ariovistus demanded that the Sequani give him more land to accommodate the Harudes people.[6][7] This demand concerned Rome because if the Sequani conceded, Ariovistus would be in a position to take all of the Sequani land and attack the rest of Gaul.[6] They did not appear to be concerned about a conflict between non-client, client and allied states. By the end of the campaign, the non-client Suebi under the leadership of the belligerent Ariovistus, stood triumphant over both the Aedui and their coconspirators. Fearing another mass migration akin to the devastating Cimbrian War, Rome, now keenly invested in the defense of Gaul, was irrevocably drawn into war.

The Helvetii was a confederation of about five related Gallic tribes that lived on the Swiss plateau, hemmed in by the mountains, and the Rhine and Rhone rivers. They began to come under increased pressure from German tribes to the north and east.[8]

By 58 BC, the Helvetii were well on their way in the planning and provisioning for a mass migration under the leadership of Orgetorix. Caesar mentions as an additional reason their not being able to in turn raid for plunder themselves due to their location. (De Bello Gallico, I, 2) They planned to travel across Gaul to the west coast, a route that would have taken them through lands of the Aedui, a Roman ally, and the Roman province of Transalpine Gaul.[8]

The Helvetii sent emissaries to neighboring tribes to negotiate peaceful transit. Orgetorix made an alliance with the Sequani chieftain Casticus and arranged the marriage of his daughter to an Aedui chieftain, Dumnorix. The three secretly planned to become kings of their respective tribes, and masters of the whole of Gaul (De Bello Gallico, I, 3). Orgetorix's personal ambitions were discovered and he was to be put on trial, with the penalty being death by fire if convicted. Orgetorix escaped with the help of his many debtors, but he died during his flight. However, the death of Orgetorix was "not without suspicion that he had decided upon death for himself"(committed suicide), as Caesar puts it (De Bello Gallico, I, 4).

Caesar dated their departure to the 28th of March, and mentions that they burned all their towns and villages so as to discourage thoughts among undecided client tribes and enemies of occupying their vacated realm. (De Bello Gallico, I, 5 and 6).

Caesar was across the Alps in Italy when he received the news. With only a single legion in Transalpine Gaul, the endangered province, he immediately hurried to Geneva and ordered a levy of several auxiliary units and the destruction of the Rhone bridge. The Helvetii sent an embassy to negotiate a peaceful passage, promising to do no harm. Caesar stalled the negotiations for fifteen days, and used the time to fortify his position with a rampart nineteen miles long and a parallel trench.(De Bello Gallico, I, 7 and 8).

Map of the Gallic Wars

When the embassy returned, Caesar refused their request and warned them that any forceful attempt to cross the river would be opposed. Several attempts were quickly beaten off. The Helvetii turned back and entered negotiations with the Sequani, and with Dumnorix of the Aeduans, for an alternate route.(De Bello Gallico, I, 8 and 9).

Leaving his single legion under the command of his second-in-command Titus Labienus, Caesar hurried to Cisalpine Gaul. Upon arrival, he took command of the three legions which were in Aquileia and enrolled two new legions, the Legio XI and the Legio XII. At the head of these five legions, he went the quickest way through the Alps, crossing territories of several hostile tribes and fighting several skirmishes en route (De Bello Gallico, I, 10).

Meanwhile, the Helvetii had already crossed the territories of the Sequani, and were busy pillaging the lands of the Aedui, Ambarri, and Allobroges. These tribes were unable to oppose them, and as Roman allies asked for Caesar's help. Caesar obliged them and surprised the Helvetii as they were crossing the river Arar (modern Saône River). Three quarters of the Helvetii had already crossed, but one quarter, the Tigurine (a Helvetian clan), was still on the east bank. Three legions, under Caesar's command, surprised and defeated the Tigurine in the Battle of the Arar. The remaining Tigurini fled to neighboring woods (De Bello Gallico, I, 11 and 12).

After the battle, the Romans built a bridge over the Saône to pursue the remaining Helvetii. The Helvetii sent an embassy led by Divico, but the negotiations failed. For a fortnight, the Romans maintained their pursuit until they ran into supply troubles. Caesar, in the meantime, sent 4,000 Roman and allied Aedui cavalry to track the Helvetii, which suffered some casualties from only 500 Helvetii cavalry ("pauci de nostris cadunt"). Apparently Dumnorix was doing everything in his power to delay the supplies. Accordingly, the Romans stopped their pursuit and headed for the Aedui town of Bibracte. The tables were turned, and the Helvetii began to pursue the Romans, harassing their rear guard. Caesar chose a nearby hill to offer battle and the Roman legions stood to face their enemies (De Bello Gallico, I, 13 to 24).

In the ensuing Battle of Bibracte, the Celts and Romans fought for the better part of the day in a hotly contested battle with the Romans eventually gaining victory. Caesar writes that "the contest long and vigorously carried on with doubtful success." The defeated Helvetii offered their surrender, which Caesar accepted. However, 6,000 men of the Helvetian clan of the Verbigeni fled to avoid capture. Upon Caesar's orders, other Gallic tribes captured and returned these fugitives, who were executed. Those who had surrendered were ordered back to their homeland to rebuild it, and the necessary supplies were organized to feed them, as they were useful as a buffer between the Romans and the northern tribes. In the captured Helvetian camp, Caesar claims that a census written in Greek was found and studied: of a grand total of 368,000 Helvetii, of which 92,000 were able-bodied men, only 110,000 survivors were left to return home (De Bello Gallico, I, 25 to 29).

In 61 BC, Ariovistus, chieftain of the Suebi tribe and a king from the Germanic peoples, resumed the tribe’s migration from eastern Germania to the Marne and Rhine region.[9] Despite the fact that this migration encroached on Sequani land, the Sequani sought Ariovistus’ allegiance against the Aedui and, in 61 BC, the Sequani rewarded Ariovistus with land following his victory in the Battle of Magetobriga.[3][4][5] Ariovistus settled the land with 120,000 of his people. When 24,000 Harudes joined his cause, Ariovistus demanded that the Sequani give him more land to accommodate the Harudes people.[6][7] This demand 'concerned' Rome because if the Sequani conceded, Ariovistus would be in a position to take all of the Sequani land and attack the rest of Gaul.[6]

Following Caesar’s victory over the Helvetii, the majority of the Gallic tribes congratulated Caesar and sought to meet with him in a general assembly.[10]Diviciacus, the head of the Aeduan government and spokesmen for the Gallic delegation, expressed concern over Ariovistus’ conquests and the hostages he had taken.[11][12] Diviciacus demanded that Caesar defeat Ariovistus and remove the threat of a Germanic invasion otherwise they would have to seek refuge in a new land.[13] Not only did Caesar have a responsibility to protect the longstanding allegiance of the Aedui, but this proposition presented an opportunity to expand Rome’s borders, strengthen the loyalty within Caesar’s army and establish him as the commander of Rome’s troops abroad.[12][14]

The senate had declared Ariovistus a "king and friend of the Roman people" in 59 BC, so Caesar could not declare war on the Suebi tribe.[15] Caesar said that he could not ignore the pain the Aedui had suffered and delivered an ultimatum to Ariovistus demanding that no German cross the Rhine, the return of Aedui hostages and the protection of the Aedui and other friends of Rome.[16] Although Ariovistus assured Caesar that the Aedui hostages would be safe as long as they continued their yearly tribute, he took the position that he and the Romans were both conquerors and that Rome had no jurisdiction over his actions.[17] With the attack of the Harudes on the Aedui and the report that a hundred clans of Suebi were trying to cross the Rhine into Gaul, Caesar had the justification he needed to wage war against Ariovistus in 58 BC.[18]

Caesar, learning that Ariovistus intended to seize Vesontio, the largest town of the Sequani, commenced marching his troops toward Vesontio. Some of Caesar’s officers held their posts for political reasons only and had no war experience. Consequently, they suffered from poor morale which threatened Caesar’s campaign. Caesar challenged the officers and their legions, saying that the only legion he could trust was the 10th. With their pride on the line, the other legions followed the 10th’s lead, determined not to be outdone. Consequently, Caesar arrived in Vesontio before Ariovistus.[19][20]

Ariovistus sent emissaries to Caesar requesting a meeting. They met under a truce at a knoll on the plain. The truce was violated when Caesar learned that German horsemen were edging towards the knoll and throwing stones at his mounted escort.[21] Two days later, Ariovistus requested another meeting. Hesitant to send senior officials, Caesar dispatched Valerius Procillus, his trusted friend, and Caius Mettius, a merchant who had successfully traded with Ariovistus. Insulted, Ariovistus threw the envoys in chains.[22][23] Ariovistus marched for two days and made camp two miles behind Caesar, thus cutting off Caesar’s communication and supply lines with the allied tribes. Unable to entice Ariovistus into battle, Caesar ordered a second smaller camp to be built near Ariovistus’ position.[24] After the camp was completed, Caesar again challenged Ariovistus and was rewarded when Ariovistus attacked the smaller camp and was repulsed.

The next morning Caesar assembled his allied troops in front of the second camp and advanced his legions in triplex acies (three lines of troops) towards Ariovistus. Each of Caesar’s five legates and his quaestor were given command of a legion. Caesar lined up on the right flank.[25] Ariovistus countered by lining up his seven tribal formations. Caesar was victorious in the battle that ensued due in large part to the charge made by Publius Crassus. As the Germans began to drive back the Roman left flank, Crassus led his cavalry in a charge to restore balance and ordered up the cohorts of the third line. As a result, the whole German line broke and began to flee.[26][27] Most of Ariovistus’ one-hundred and twenty thousand men were killed. He and what remained of his troops escaped and crossed the Rhine, never to engage Rome in battle again. The Suebi camped near the Rhine returned home. Caesar was victorious.[28][29]

In 57 BC Caesar once again intervened in an intra-Gallic conflict, marching against the Belgae, who inhabited the area roughly bounded by modern-day Belgium. The Belgae had recently attacked a tribe allied with Rome and before marching out with his army to meet them, Caesar ordered the Remi and other neighbouring Gauls to investigate the Belgae's actions.[30] His army suffered a surprise attack in the battle of the Sabis while it was making camp near the river Sambre.

The Nervii advanced so quickly that Caesar didn't have the time to organise his forces and nearly suffered a humiliating defeat. Caesar admits to losing all of his standards and most his centurions dead or felled by wounds. He himself was forced to take up a shield and personally rally his forces which were then threatened with envelopment and massacre. The strong stand by the X legion and the prompt arrival of reinforcements enabled Caesar to regroup, redeploy and eventually repulse the Nervii once the Atrebates and Viromandui were put to flight.[8]

Caesar remarked that the warlike Nervii refused to yield their ground even after the Atrebates and Virumandui had been put into disarray. When finally surrounded by Roman reinforcements the Nerviians continued fighting as a pitiless hail of missiles rained down on them from the many archers and peltasts Caesar had brought from overseas. The peltasts, slingers and archers were brought for the specific purpose of confounding the Gallic proclivity for shield wall tactics, mass attack and individual close combat.

The Nervii were especially renowned for skills at warfare. They ruled and subsisted by warfare and by taxing their dependent and client tribes while adhering to a heroic hoplitic tradition. These Gallic conventions were something Caesar exploited as often as he could.

Together with Caesar's prudent and unabashed use of fixed projectile weapons like the "scorpion" and light ballista, the archers and peltasts took a heavy toll on the densely packed Nervii, who themselves shunned all projectile weapons but the lance. It is recorded in Caesar's war commentaries that as the battle raged, the Nervii caught Roman javelins in flight and hurled them back at legionnaires and that although all were eventually slain, not one of the Nervii was seen to flee. As the grim fighting wore on, the Nervii refused to yield and mounds of the fallen formed ramparts and Boduognatus' fighters fought from atop these hills of dead, clashing with the pressing Roman ranks again and again.

The skill with which the veteran Roman legions executed their well practiced pilum barrage and gladius and scutum counter-attacks together with the prudent use of missile weapons was instrumental in defeating the skillful and daring Nervii and associated Belgae.

The Belgae suffered heavy losses and eventually surrendered when faced with the destruction of their towns. The Nervii were severely mauled and forced to flee; thereby all former client tribes surrendered to Caesar or likewise fled. Their absence gave Caesar control of most of what is now Belgium.

The following year, 56 BC, Caesar turned his attention to the tribes of the Atlantic seaboard, notably the Veneti tribe in Armorica (modern Brittany), who had assembled a confederacy of anti-Roman tribes. The Veneti were a seafaring people and had built a sailing fleet in the Gulf of Morbihan, requiring the Romans to build galleys and undertake an unconventional land and sea campaign. Eventually Caesar was able to subdue these tribes after a protracted land and sea campaign.

Caesar took his forces across the Rhine in 55 BC in a punitive expedition against the Germans, though the Suebi, against whom the expedition was mounted, were never engaged in battle. That same year, he then crossed the English Channel with two legions on his ships to mount a similar expedition against the Britons. The British adventure nearly ended in defeat when bad weather wrecked much of their fleet and the unfamiliar sight of massed chariots caused confusion among their forces. Caesar did manage to secure a promise of hostages, though only two of them were actually sent. He withdrew, but returned in 54 BC with a much larger force that successfully defeated the powerful Catuvellauni, and forced them to pay tribute to Rome. The expeditions had little lasting effect, but were great propaganda victories for Caesar, keeping him in the public eye at home.

The campaigns of 55 BC and early 54 BC have caused controversy for many centuries. They were controversial even at the time among Caesar's contemporaries, and especially among his political opponents, who decried them as a costly exercise in personal aggrandizement. In modern times, commentators have been sharply divided between critics of Caesar's nakedly imperialist agenda and defenders of the benefits that the expansion of Roman power subsequently wrought in Gaul.

Roman silver denarius with the head of captive Gaul 48 BC, following the campaigns of Caesar.

Discontent among the subjugated Gauls prompted a major uprising in the winter of 54–53 BC, when the Eburones of north-eastern Gaul rose in arms under their leader Ambiorix. Fifteen Roman cohorts were wiped out at Atuatuca Tungrorum (modern Tongeren in Belgium) and a garrison commanded by Quintus Tullius Cicero narrowly survived after being relieved by Caesar in the nick of time. The rest of 53 BC was occupied with a punitive campaign against the Eburones and their allies, who were said to have been all but exterminated by the Romans.

The uprising was, however, merely the prelude to a much bigger campaign led by Vercingetorix, chief of the Arverni tribe of central Gaul, who successfully united many Gallic tribes and states under his leadership. Recognizing that the Romans had an upper hand on the battlefield due to their panoply and training, he declined to give battle against them and instead fought a "scorched earth" campaign to deprive them of supplies. Caesar hurriedly returned from Italy to take charge of the campaign, pursuing the Gauls and capturing the town of Avaricum (modern city of Bourges) but suffering a defeat at Gergovia. Vercingetorix, instead of staying mobile and in the open, chose to hold out at Alesia (see Battle of Alesia). Caesar successfully besieged him and beat off a huge Gallic relief force who ran out of food and had to disperse. This effectively marked the end of the Gallic Wars, although mopping-up actions took place throughout 51 BC. A number of lesser rebellions took place subsequently, but Roman control of Gaul was not seriously challenged again until the second century AD.

The primary historical source for the Gallic Wars is Caesar's Commentarii de Bello Gallico in Latin, which is one of the best surviving examples of unadorned Latin prose. It has consequently been a subject of intense study for Latinists, and is one of the classic prose sources traditionally used as a standard teaching text in modern Latin education.

The Gallic Wars have become a popular setting in modern historical fiction, especially that of France and Italy. Claude Cueni wrote a semi-historical novel, The Caesar's Druid, about a fictional Celtic druid, servant of Caesar and recorder of Caesar's campaigns. Morgan Llewelyn also wrote a book, Druids, about a Celtic druid who assisted Vercingetorix in his campaign against Julius Caesar. Similarly, Norman Spinrad's, The Druid King, follows the campaigns from Vercingetorix's perspective.[31] In addition, the comicAstérix is set shortly after the Gallic Wars, where the titular character's village is the last holdout in Gaul against Caesar's legions.

S.J.A. Turney's series Marius' Mules[citation needed] tells the story of the Gallic Wars with a fictional protagonist, Marcus Falerius Fronto. Each volume is based on a volume of 'Commentarii de Bello Gallico'.

The concept album Helvetios by Swiss folk metal band, Eluveitie, tells the story of the Gallic Wars through the eyes of the Helvetii.

^That the Balkans were Caesar's original target is argued by several scholars, including: Penguin Classics The Conquest of Gaul: "Introduction", chapter 3 "The course of the war"[clarification needed], Adrian Goldsworthy, In the Name of Rome, chapter 8 "Caesar in Gaul". It is suggested by the provinces Caesar initially wanted for himself (Cisalpine Gaul and Illyricum) and supported by the initial placement of three of his four legions in Aquileia.